Integration of oxy-fuel and air-fuel combustion

ABSTRACT

A furnace is heated by a burner that can be selectively operated by either air-fuel or oxy-fuel combustion. The burner comprises a conduit for fuel, a conduit for air, a conduit for oxidant, and control means for regulating flow through the air and oxidant conduits. An air-fuel fired furnace can be modified by addition of the oxidant and fuel conduits and the control means for regulating flow through air and oxidant conduits.

FIELD OF THE INVENTION

The present invention relates to combustion of fuel in a furnace, andespecially in a furnace used to heat solid and liquid materials and/orto melt solid materials, as the materials are held in or passing throughthe furnace.

BACKGROUND OF THE INVENTION

Many industrial processes require heating material to elevatedtemperatures, on the order of 1000° F. or higher. Examples are numerousbut include heating or reheating steel prior to its being worked in amill, and melting glassmaking materials to form a glassmelt from whichglass products are formed.

In many of these applications the heat is applied to the material in afurnace in which the material has been placed, or through which thematerial is passed. The heat is obtained by combustion within thefurnace, at one or more burners where fuel is burned to produce heat ofcombustion.

In many furnaces the burner or burners combust fuel with air, which ofcourse contains the oxygen needed for the combustion. Such combustion istermed “air-fuel combustion” and burners at which air-fuel combustionoccurs are termed “air-fuel burners”. In many other applications theburner or burners combust fuel with a gaseous oxidant that containsoxygen in a concentration higher than that of air, ranging from 25 vol.% to 99 vol. % depending on the application and other considerationssuch as (but not limited to) economics, the higher temperature at whichthe combustion (termed “oxy-fuel combustion”) occurs, and theopportunity to generate a smaller amount of nitrogen oxides. Oxy-fuelcombustion often requires the use of burners (termed “oxy-fuel burners”)that are adapted for oxy-fuel combustion, in particular in their abilityto withstand the higher combustion temperatures obtained in oxy-fuelcombustion.

Some applications attempt to use both air-fuel combustion and oxy-fuelcombustion. One example occurs in steel reheating furnaces, in which apiece (slab, bloom or billet) of steel is passed through a furnacewherein the piece is heated first by the heat provided from one or moreair-fuel burners and then (as it continues its passage through thefurnace) by heat provided from one or more oxy-fuel burners. Inaddition, in some industrial heating processes the advantages ofoxy-fuel combustion have led operators to remove air-fuel burners andreplace them with oxy-fuel burners or add additional zones composed ofoxy-fuel burners.

There remains a need, however, to be able to selectively andalternatingly obtain the benefits of air-fuel combustion and oxy-fuelcombustion, without having to undergo the expense and lost time thatwould be encountered in repeatedly removing air-fuel burners, replacingthem with oxy-fuel burners, and then replacing the oxy-fuel burners withair-fuel burners, and continuing to repeat the cycle.

BRIEF SUMMARY OF THE INVENTION

The present invention, in one aspect, is combustion apparatus comprising

(a) a furnace enclosing a combustion zone and having at least one burnerthrough a wall of the furnace to which air is fed through an air conduitand fuel is fed through a burner fuel conduit from outside the furnaceto be combusted at the burner within the combustion zone;

(b) an oxidant conduit through which oxidant can be fed into the furnacefrom outside the furnace; and

(c) control means that regulates the flow of oxidant through the oxidantconduit and the flow of air through the air conduit such that the ratioof air flow to oxidant flow can be controlled;

wherein the oxidant conduit and the burner fuel conduit are orientedwith respect to each other so that the oxidant conduit feeds oxidantinto an oxidant mixing zone in the combustion zone and the burner fuelconduit feeds fuel into a fuel reaction zone in the combustion zonewhich is segregated from the oxidant mixing zone.

Another aspect of the present invention is a burner apparatus comprising

(a) a burner to which air is fed through an air conduit and fuel is fedthrough a burner fuel conduit to be combusted at the burner;

(b) an oxidant conduit through which oxidant can be fed to the burner;and

(c) control means that regulates the flow of oxidant through the oxidantconduit and the flow of air through the air conduit such that the ratioof air flow to oxidant flow can be controlled;

wherein the oxidant conduit and the burner fuel conduit are orientedwith respect to each other so that the oxidant conduit feeds oxidantinto an oxidant mixing zone in the combustion zone and the burner fuelconduit feeds fuel into a fuel reaction zone in the combustion zonewhich is segregated from the oxidant mixing zone.

Another aspect of the present invention is a method for retrofitting anair-fired furnace, comprising

(a) providing a furnace enclosing a combustion zone and having at leastone burner through a wall of the furnace to which air is fed through anair conduit and fuel is fed through a burner fuel conduit from outsidethe furnace to be combusted at the burner within the combustion zone;

(b) providing an oxidant conduit through which oxidant can be fed intothe furnace from outside the furnace;

(c) providing control means that regulates the flow of oxidant throughthe oxidant conduit and the flow of air through the air conduit suchthat the ratio of air flow to oxidant flow can be controlled; and

(d) orienting the oxidant conduit with respect to the burner fuelconduit so that the oxidant conduit feeds oxidant into an oxidant mixingzone in the combustion zone and the burner fuel conduit feeds fuel intoa fuel reaction zone in the combustion zone which is segregated from theoxidant mixing zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a burner with which the presentinvention can be practiced.

FIG. 2 is a cross-sectional view of one embodiment of the presentinvention.

FIG. 3 is a plan view of a wall of a furnace showing the embodiment ofthe invention that is shown in FIG. 2.

FIG. 4 is a plan view of a wall of a furnace showing another embodimentof the present invention.

FIG. 5 is a plan view of a wall of a furnace showing yet anotherembodiment of the present invention.

FIG. 6 is a plan view of a wall of a furnace showing another embodimentof the present invention.

FIG. 7 is a schematic representation of combustion in one embodiment ofthe invention.

FIG. 8 is a schematic representation of combustion in another embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be practiced in any furnace of conventional design,which will typically comprise an enclosure within which combustion athigh temperature takes place. The enclosure is typically lined withmaterial such as refractory furnace brick or the equivalent that canwithstand temperatures of several thousand degrees which are generatedwithin the furnace enclosure. Preferably, the floor, all sides, and theroof of the furnace are all lined with such material. Examples offurnaces with which this invention can be practiced include steelreheating furnaces and other furnaces through which solid material ispassed to be heated, as well as glass melting furnaces and otherfurnaces in which material fed to the furnace is to be melted or to bemaintained in a molten state.

The desired high temperature is established within the furnace bycombustion carried out at one or more burners. FIG. 1 depicts onetypical burner currently employed to combust fuel and air to establishthe high temperature within a furnace. Burner 1 is located so that itopens through wall 2 of the furnace toward combustion zone 3. Burner 1includes fuel passage 4 and air passages 5. Fuel is fed through fuelpassage 4 into combustion zone 3 inside the furnace and combusts withthe oxygen contained in air that is fed through air passages 5, therebyestablishing a flame and providing heat of combustion to combustion zone3 and throughout the interior of the furnace.

Suitable fuels for this air-fuel combustion include gaseoushydrocarbons, such as natural gas and methane, byproduct gases producedin steel mills, such as coke oven gas and blast furnace gas, mixtures ofthese gaseous fuels, as well as liquid fuels such as atomized fuel oil,and solid fuels such as pulverized coal. The fuel and the air aresupplied through their respective passages 4 and 5 by suitable meansconnected to sources thereof, all by conventional technology quitefamiliar to those of ordinary skill in this field.

Apparatus, indicated schematically as 13 in FIGS. 1 and 2, regulates theflow rate of fuel into and through fuel passage 4, and regulates whetherfuel is permitted to flow into and through fuel passage 4. Otherapparatus, indicated schematically as 16 in FIGS. 1 and 2, regulates theflow rate of combustion air into and through air passages 5.

The present invention can add to burners that combust fuel in anair-fuel mode of combustion the capability to selectively combust fuelin an oxy-fuel mode of combustion. This capability can be added by,among other things, providing a way to feed oxidant having a higheroxygen content than the oxygen content of air into combustion zone 3.Preferably, the oxygen has an oxygen concentration of at least 25 vol.%, and more preferably at least 90 vol. %. A preferred manner ofcarrying out this feeding is shown in FIG. 2, which depicts oxidantlance 14 that has been situated in an air passage 5. Oxidant lance 14 isfed by suitable apparatus, indicated schematically as 15 in FIG. 2,which supplies the oxidant and can controllably regulate the flow rateof oxidant into and through lance 14 and can also controllably regulatewhether or not oxidant is even permitted to flow into and throughoxidant lance 14.

The present invention can be operated so that in the oxy-fuel combustionmode the fuel that is combusted is the same as the fuel that iscombusted in the air-fuel combustion mode. In such cases, the fuel canbe supplied through fuel passage 4. Alternatively, such as when the fuelthat is combusted in the oxy-fuel combustion mode is different from thefuel that is combusted in the air-fuel combustion mode, or when the fuelfed in the oxy-fuel combustion mode must be fed at a higher flow rate,the fuel for oxy-fuel combustion is fed through a second fuel conduit.One such second fuel conduit is shown in FIG. 2 as fuel lance 11, whichis situated within fuel passage 4 so that the orifice of lance 11 issufficiently close to the opening of fuel passage 4 that a flame formedupon combustion of fuel that is fed from the end of fuel lance 11 iswell supported and extends into combustion zone 3. Fuel is fed into fuellance 11 from a source, indicated schematically as 12 in FIG. 2, whichalso controls the flow rate of fuel into and through fuel lance 11 andcontrols whether or not fuel can flow into and through fuel lance 11 aswell as the ratio of fuel flow through fuel lance 11 and through fuelpassage 4.

As is described further below, in the oxy-fuel combustion mode therelative momentum of the fuel flow and the oxidant flow needs to bemanaged. In most cases where the oxidant conduit is within the burner,the second fuel conduit will be required that is capable of feeding thefuel into the combustion zone 3 at the requisite higher velocity. If NOxformation from combustion in the furnace is not an issue, then theexisting fuel conduit can be employed with the oxidant conduit describedherein. If NOx formation is an issue, then the second fuel conduit couldbe integrated into the air-fuel burner through its fuel conduit if it issuitably sized, or through a hole leading into the combustion airconduit, or outside the burner through a hole in the wall of the furnaceas shown in FIG. 5.

FIG. 3 is a view of the front of the burner depicted in FIG. 2 seen frominside the furnace enclosure. There it can be seen that fuel lance 11 islocated within fuel passage 4, and oxidant lance 14 is located withinair passage 5.

Other embodiments that accomplish the same objectives of the inventioncan also be employed. Indeed, depending on the configuration of theair-fuel burner, and depending on the available space in the immediatearea outside the burner, other configurations may be preferable for easeof construction and operation.

FIG. 4 depicts one such alternative embodiment, wherein the burner andthe fuel lance 11 serving as the second fuel conduit are as describedwith respect to FIGS. 2 and 3, but the oxidant is supplied through lance14 which discharges oxidant into combustion zone 3 within the furnacefrom a point adjacent to the burner but outside the burner (meaning notwithin the space bounded by the external surface of the burner where itopens toward combustion zone 3.

FIG. 5 depicts another alternative embodiment, wherein the oxidant issupplied to the combustion zone through lance 14 which is located in airconduit 5, and fuel lance 11 serving as the second fuel conduitdischarges fuel into combustion zone 3 within the furnace from a pointadjacent to the burner but outside the burner.

FIG. 6 depicts another alternative embodiment, wherein both the oxidantlance 14 and the lance 11 serving as the second fuel conduit are locatedin air conduit 5.

The lance or other apparatus by which fuel is to be fed into combustionzone 3 in the oxy-fuel mode of operation, and the lance or other devicethrough which oxidant is fed in to combustion zone 3 or the oxy-fuelmode of operation, must be oriented with respect to each other so thatthe oxidant mixing zone, into which the oxidant is fed as describedhereinbelow, and the fuel reaction zone, into which the fuel is to befed, are segregated (i.e., physically distinct from each other) withincombustion zone 3. The feeding of the oxygen and fuel, and the operationof the burner when it is in the oxy-fuel mode of operation, should becarried out in accordance with the description contained in U.S. Pat.No. 5,076,779, the entire content of which is hereby incorporated hereinby reference. In particular, the oxidant is injected into combustionzone 3 with velocity sufficient to entrain or mix furnace gases that arein combustion zone 3 with the injected oxidant. The furnace gasescomprise ambient gases which infiltrate into the combustion zone, andgases from the oxidant mixture and fuel reaction mixture. Generally thevelocity of the oxidant will be at least 200 feet per second andpreferably is within the range of 250 to sonic velocity (1,070 feet persecond at 70° F.). The velocity of the oxidant is such that sufficientfurnace gases mix with the injected oxidant to dilute the oxygenconcentration of the injected oxidant so that an oxidant mixture isproduced within the oxidant mixing zone having an oxygen concentrationof not more than 10 vol. % and preferably not more than 5 vol. %. Whenpure oxygen or oxygen-enriched air is used as the oxidant, higherentrainment of the furnace gas is required to reduce the oxygenconcentration to the desired lower levels. No combustion reaction takesplace in this zone because the furnace atmosphere entrained into theoxidant jet is substantially free of fuel.

The furnace gases mix with or are entrained into the oxidant due to theturbulence or the aspiration effect caused by the high velocity of theoxidant stream being fed into the oxidant mixing zone. The resultingoxidant mixture, containing a significantly lower concentration ofoxygen than was present in the injected oxidant, flows out from theoxidant mixing zone and serves to form part of the atmosphere withincombustion zone 3. That is, the oxidant mixture provides additionalfurnace gases to combustion zone 3.

When fuel is injected into combustion zone 3 during the oxy-fuel mode ofoperation of the invention, furnace gases from the atmosphere withincombustion zone 3 flow into and mix with the fuel stream due to theturbulence caused by the fuel stream injection, and the oxygen withinthe furnace gases combusts with the fuel in the fuel reaction zone.Depending on the amount of air delivered through air conduit 5 and therelative location of fuel lance 11, a small amount of fuel may reactwith the air supplied via air conduit 5 in a combustion zone of thefurnace prior to the main combustion zone 3.

The temperature within the combustion zone 3 should exceed 1400° F. astemperatures below 1400° F. can result in flame instabilities. The fuelreacts with oxygen molecules in the furnace gases spontaneously, as thetemperature of the furnace gas is above the auto-ignition temperature ofthe fuel and oxygen. However, since the oxygen concentration isrelatively low, the flame temperature is kept relatively low due to thepresence of large amounts of non-reacting molecules such as carbondioxide, water vapor, and molecular nitrogen in the fuel reaction zone.The combustion under these conditions in the fuel reaction zone producesheat of combustion and combustion reaction products such as carbondioxide and water vapor but produces very little nitrogen oxides. Theactual amount of nitrogen oxides produced varies with each particularsituation and will depend on factors such as the furnace gastemperature, nitrogen concentration in the combustion zone and theresidence time.

The resulting fuel mixture including the combustion reaction productsflows out of the fuel reaction mixture and serves to form part of theatmosphere within combustion zone 3 thus providing additional furnacegases to the combustion zone. Within the fuel reaction zone, the fuelundergoes substantially complete combustion so that there is nosignificant amount of uncombusted or incompletely combusted fuel in thecombustion zone outside of the fuel reaction zone.

It is important in the practice of the oxy-fuel combustion mode of thisinvention that the oxidant mixing zone and the fuel reaction zone aremaintained separate from each other (or “segregated”) within combustionzone 3. In this way, combustion is restricted primarily to the fuelreaction zone and under conditions which dampen formation of nitrogenoxides (“NOx”). Although various steps of this mode of combustion aredescribed in sequence, those skilled in the art will appreciate that thesteps of this method are conducted simultaneously and continuously.

The oxidant mixing zone and the fuel reaction zone can be maintainedsegregated as desired, by positioning the injection points (that is, theends of lances 11 and 14, for example) and orienting the injectiondirections, of the fuel and oxidant so as to avoid integration andoverlap thereof prior to the requisite dilution of the oxidant withinthe oxidant mixing zone and the requisite substantially completecombustion of the fuel within the fuel reaction zone.

The fuel and the oxidant are fed into the combustion zone 3 in a mannerto achieve sufficient mixing within combustion zone 3 so that thecombustion zone atmosphere outside of the oxidant mixing zone and of thefuel reaction zone is substantially homogeneous. In a particularlypreferred embodiment, the fuel and the oxidant are injected intocombustion zone 3 in a manner to promote a recirculating pattern offurnace gases within combustion zone 3. This recirculating patterncontributes to improved temperature distribution and gas homogeneitywithin the combustion zone 3 and improves the mixing within the oxidantmixing zone and within the fuel reaction zone, resulting in smoothercombustion and retarding formation of NOx. With optimum furnace gasrecirculation within combustion zone 3, the composition of the flue gastaken out of the combustion zone is substantially the same as thecomposition of the atmosphere at points within combustion zone 3 outsideof the oxidant mixing zone and fuel reaction zone. This recirculationpattern also promotes the entrainment of the furnace gases downstream ofthe fuel reaction zone into the oxidant stream and the entrainment ofthe furnace gases downstream of the oxidant mixing zone into the fuelstream.

It is particularly preferred to feed the oxidant stream and the fuelstream, in the oxy-fuel combustion mode of operation of the invention,at high velocities and away from each other so that the oxidant mixingzone and the fuel reaction zone do not overlap. Preferably, the ratio ofthe fuel stream momentum flux to the oxidant stream momentum flux shouldbe within 1:5 to 5:1 when injected from relatively close proximity, suchas in the embodiments depicted in FIGS. 3-6.

FIGS. 7 and 8 illustrate two embodiments of the oxy-fuel mode ofcombustion that can be practiced. The letter “O” designates an oxidantmixing zone and the letter “F” designates the fuel mixing zone. Thearrows pointed toward oxidant mixing zone “O” depict furnace gases beingdrawn toward and into the oxidant mixing zone, and the arrows pointedtoward fuel reaction zone “F” depict furnace gases flowing toward andinto the fuel reaction zone.

The adaptation of an air-fuel burner into a burner which is capable ofselectively carrying out air-fuel combustion and oxy-fuel combustion isaided by providing suitable controls so that the operator cancontrollably switch between an air-fuel combustion mode and an oxy-fuelcombustion mode at the same burner. Providing this capability requirescontrols which can controllably minimize or in the limit, shut off orturn on, the flow of air through the air passages, and which cancontrollably shut off or turn on the flow of oxidant through the oxidantlance or other unit by which oxidant is fed to combustion zone 3.Preferably, the controls also permit regulation of the flow rates of thecombustion air, and the flow rate of oxidant, through their respectiveconduits. In its simplest mode, the control mechanism can comprisesimply a regulating valve controlling the flow of oxidant to combustionzone 3, and a regulating valve controlling the flow of air to the airpassages of the burner. In most embodiments, one will desire to shut offone such flow completely when the other such flow is to be turned on.Commercially available oxygen supply equipment typically has doubleblock valves (for safety), flow measurement devices, pressure switchesand other instrumentation with which this level of control can befacilitated.

In addition, in those embodiments in which the same fuel is used whetherthe combustion is air-fuel or oxy-fuel, no additional controls need tobe provided so long as controls were already present to regulate theflow rate of fuel through the burner into combustion zone 3. However, inthose embodiments wherein a different fuel, or a different fuel feedconduit, is provided depending on whether the combustion is air-fuel oroxy-fuel, then controls should be provided that permit the operator toshut off the flow of fuel associated with the air-fuel combustion whenthe oxy-fuel combustion mode is to be operated, and to shut off the flowof fuel associated with the oxy-fuel combustion when the air-fuelcombustion mode is to be operated. However, even when the same fuel iscombusted in the air-fuel and oxy-fuel modes, the oxy-fuel mode usuallyrequires a higher velocity fuel flow rate. Accordingly, the fuelsupplied from the fuel delivery and metering system that is in place forsupplying fuel to the fuel conduit for feeding fuel to the air-fuelburner for air-fuel combustion (e.g. typically, low velocity fuelsupply) is switched to the second fuel conduit that is used for feedingfuel for oxy-fuel combustion (i.e. to the burner, or to a conduit 11, orto a separate opening 11 as shown for instance in FIG. 5) This providesthe benefit that the existing fuel supply and metering system ismaintained and simply switched between conduits.

The controls preferably permit a base flow of air through the airconduit, even in the oxy-fuel combustion mode wherein oxidant is beingfed and combusted. The controls give the operator the ability togradually, controllably increase the ratio of the oxidant flow rate tothe air flow rate until the desired combustion conditions areestablished.

When the air-fuel burner has been fitted as described herein, to providethe capability to controllably carry out oxy-fuel combustion andair-fuel combustion at the same burner, and to controllably alternate asdesired between air-fuel combustion and oxy-fuel combustion at the sameburner, the resultant apparatus and its capability provide severalsignificant advantages to the operator. One such advantage is thatenergy efficiency can be improved. That is, fuel consumed for a givenamount of furnace output is improved, and the fuel costs can be reducedeven taking into account the cost of the oxygen in the oxidant that isconsumed. Another advantage is that productivity, in the sense of theamount of furnace output (such as the amount of steel that is reheated)in a given unit of time), is improved. Depending on the characteristicsof the furnace before retrofitting as described herein, this improvementcan be attributed to the fact that combustion with oxidant having anelevated oxygen content relative to air can overcome the furnace'slimitations in the amount of combustion air that it could be fed in theair-fuel combustion mode, and/or to the reduction in the volume of fluegas that must be discharged through the flue (since this flue gas willcontain less nitrogen than flue gas generated in air-fuel combustion).

1. Combustion apparatus comprising (a) a furnace enclosing a combustionzone and having at least one burner through a wall of the furnace towhich air is fed through an air conduit and fuel is fed through a burnerfuel conduit from outside the furnace to be combusted at the burnerwithin the combustion zone; (b) an oxidant conduit through which oxidantcan be fed into the furnace from outside the furnace; and (c) controlmeans that regulates the flow of oxidant through the oxidant conduit andthe flow of air through the air conduit such that the ratio of air flowto oxidant flow can be controlled; wherein the oxidant conduit and theburner fuel conduit are oriented with respect to each other so that theoxidant conduit feeds oxidant into an oxidant mixing zone in thecombustion zone and the burner fuel conduit feeds fuel into a fuelreaction zone in the combustion zone which is segregated from theoxidant mixing zone.
 2. Combustion apparatus according to claim 1wherein the oxidant conduit feeds oxidant into the furnace from withinthe burner.
 3. Combustion apparatus according to claim 1 wherein theoxidant conduit feeds oxidant into the furnace from an opening that isnot within a burner.
 4. Combustion apparatus according to claim 1further comprising a second fuel conduit through which fuel is fed fromoutside the furnace to be combusted within the combustion zone. 5.Combustion apparatus according to claim 4 wherein the oxidant conduitfeeds oxidant into the furnace from within the burner.
 6. Combustionapparatus according to claim 5 wherein the second fuel conduit feedsfuel into the furnace from within the burner.
 7. Combustion apparatusaccording to claim 5 wherein the second fuel conduit feeds fuel into thefurnace from an opening that is not within a burner.
 8. Combustionapparatus according to claim 4 wherein the oxidant conduit feeds oxidantinto the furnace from an opening that is not within a burner. 9.Combustion apparatus according to claim 8 wherein the second fuelconduit feeds fuel into the furnace from within the burner. 10.Combustion apparatus according to claim 8 wherein the second fuelconduit feeds fuel into the furnace from an opening that is not within aburner.
 11. Burner apparatus comprising (a) a burner to which air is fedthrough an air conduit and fuel is fed through a burner fuel conduit tobe combusted in a combustion zone at the burner; (b) an oxidant conduitthrough which oxidant can be fed to the burner; and (c) control meansthat regulates the flow of oxidant through the oxidant conduit and theflow of air through the air conduit such that the ratio of air flow tooxidant flow can be controlled; wherein the oxidant conduit and theburner fuel conduit are oriented with respect to each other so that theoxidant conduit feeds oxidant into an oxidant mixing zone in thecombustion zone and the burner fuel conduit feeds fuel into a fuelreaction zone in the combustion zone which is segregated from theoxidant mixing zone.
 12. Burner apparatus according to claim 1 1 whereinthe oxidant conduit feeds oxidant into the furnace from within theburner.
 13. Burner apparatus according to claim 11 wherein the oxidantconduit feeds oxidant into the furnace from an opening that is notwithin a burner.
 14. Burner apparatus according to claim 11 furthercomprising a second fuel conduit through which fuel is fed to becombusted at the burner.
 15. Burner apparatus according to claim 14wherein the oxidant conduit feeds oxidant into the furnace from withinthe burner.
 16. Burner apparatus according to claim 15 wherein thesecond fuel conduit feeds fuel into the furnace from within the burner.17. Burner apparatus according to claim 15 wherein the second fuelconduit feeds fuel into the furnace from an opening that is not within aburner.
 18. Burner apparatus according to claim 14 wherein the oxidantconduit feeds oxidant into the furnace from an opening that is notwithin a burner.
 19. Burner apparatus according to claim 18 wherein thesecond fuel conduit feeds fuel into the furnace from within the burner.20. Burner apparatus according to claim 18 wherein the second fuelconduit feeds fuel into the furnace from an opening that is not within aburner.
 21. A method for retrofitting an air-fired furnace, comprising(a) providing a furnace enclosing a combustion zone and having at leastone burner through a wall of the furnace to which air is fed through anair conduit and fuel is fed through a burner fuel conduit from outsidethe furnace to be combusted at the burner within the combustion zone;(b) providing an oxidant conduit through which oxidant can be fed intothe furnace from outside the furnace; (c) providing control means thatregulates the flow of oxidant through the oxidant conduit and the flowof air through the air conduit such that the ratio of air flow tooxidant flow can be controlled; and (d) orienting the oxidant conduitwith respect to the burner fuel conduit so that the oxidant conduitfeeds oxidant into an oxidant mixing zone in the combustion zone and theburner fuel conduit feeds fuel into a fuel reaction zone in thecombustion zone which is segregated from the oxidant mixing zone.
 22. Amethod according to claim 21 wherein the oxidant conduit feeds oxidantinto the furnace from within the burner.
 23. A method according to claim21 wherein the oxidant conduit feeds oxidant into the furnace from anopening that is not within a burner.
 24. A method for retrofitting anair-fired furnace, comprising (a) providing a furnace enclosing acombustion zone and having at least one burner through a wall of thefurnace to which air is fed through an air conduit and fuel is fedthrough a burner fuel conduit from outside the furnace to be combustedat the burner within the combustion zone; (b) providing an oxidantconduit through which oxidant can be fed into the furnace from outsidethe furnace; (c) providing control means that regulates the flow ofoxidant through the oxidant conduit and the flow of air through the airconduit such that the ratio of air flow to oxidant flow can becontrolled; (d) providing a second fuel conduit through which fuel isfed from outside the furnace to be combusted within the combustion zone,and (e) orienting the oxidant conduit with respect to at least one ofthe burner fuel conduit and the second fuel conduit so that the oxidantconduit feeds oxidant into an oxidant mixing zone in the combustion zoneand said fuel conduit feeds fuel into a fuel reaction zone in thecombustion zone which is segregated from the oxidant mixing zone.
 25. Amethod according to claim 24 wherein the oxidant conduit feeds oxidantinto the furnace from within the burner.
 26. A method according to claim25 wherein the second fuel conduit feeds fuel into the furnace fromwithin the burner.
 27. A method according to claim 25 wherein the secondfuel conduit feeds fuel into the furnace from an opening that is notwithin a burner.
 28. A method according to claim 24 wherein the oxidantconduit feeds oxidant into the furnace from an opening that is notwithin a burner.
 29. A method according to claim 28 wherein the secondfuel conduit feeds fuel into the furnace from within the burner.
 30. Amethod according to claim 28 wherein the second fuel conduit feeds fuelinto the furnace from an opening that is not within a burner.